Respiratory Testing for Guillain-Barré Syndrome
Measure vital capacity (VC), maximum inspiratory pressure (MIP), and maximum expiratory pressure (MEP) immediately and serially every 2-4 hours, along with the single breath count test, as these are the essential respiratory tests for GBS patients with respiratory involvement. 1, 2
Primary Bedside Respiratory Tests
Single Breath Count Test
- The inability to count to ≤19 (or ≤15) in a single breath is the most ominous bedside sign for imminent respiratory failure and should trigger immediate ICU admission 2
- Each counted number approximately equals 116 mL of vital capacity, making this a practical surrogate for VC measurement 2
- This test directly predicts the need for mechanical ventilation 2
The "20/30/40 Rule" - Objective Measurements
Apply these thresholds to identify patients at imminent risk of respiratory failure 1, 2:
- Vital capacity <20 mL/kg 1, 3
- Maximum inspiratory pressure <30 cmH₂O 1, 3
- Maximum expiratory pressure <40 cmH₂O 1, 3
Additional Critical Threshold
- A reduction of more than 30% in VC, MIP, or MEP from baseline indicates high risk for progression to respiratory failure 3
Serial Monitoring Protocol
- Perform respiratory function measurements every 2-4 hours in patients with suspected respiratory involvement 2
- Up to 22% of GBS patients require mechanical ventilation within the first week of admission, and respiratory failure can develop rapidly without obvious dyspnea 1, 4
- Serial measurements are mandatory because the pattern of respiratory decline is inherently unpredictable, though these tests allow detection of those at risk 3
Clinical Assessment Alongside Testing
Assess for these physical signs that indicate respiratory compromise 1:
- Use of accessory respiratory muscles 1
- Bulbar weakness (independently predicts respiratory failure with OR 7.6) 5
- Neck muscle weakness with MRC score ≤3 (independently predicts respiratory failure with OR 9.2) 5
- Bilateral facial weakness 3
What NOT to Rely On
Do not rely on pulse oximetry or arterial blood gases as early indicators of respiratory failure - these are late findings and will miss the window for elective intubation 2, 4
Arterial blood gases should only be obtained if respiratory compromise is already suspected based on the above measurements, not as a primary screening tool 1
ICU Admission Triggers
Admit to ICU immediately when 1, 2:
- Single breath count ≤19
- VC <20 mL/kg
- MIP <30 cmH₂O
- MEP <40 cmH₂O
- Rapid progression of weakness
- Severe bulbar dysfunction or diminished cough reflex
Critical Pitfall
The most dangerous pitfall is that respiratory failure can develop rapidly without obvious clinical signs of dyspnea 1, 4. Therefore, objective measurements trump clinical impression, and serial testing is non-negotiable even in patients who appear comfortable.